As described in European patent publication No. 3,391 of H. M.sup.c Master et al, in the patent documents cited therein, in German Pat. No. 848,237 of J. Gorner, and in U.S. Pat. Nos. 4,361,432 and 4,364,765, it is known to make a shaped piece of tempered glass in a two-stage process. In the first stage the normally planar glass sheet, which is heated above its softening or deformation point is bent or deformed into a nonplanar shape, by differential pressure acting against it or by simple mechanical shaping. Then the still-soft shaped sheet is cooled rapidly, with blasts of cool air on its surface to prestress and temper the glass. The surface of such glass is very hard, the glass is very strong, and when broken it disintegrates into small relatively smooth pieces that pose little danger of injury.
The quenching unit is normally an upper and a lower array of nozzles, the upper ones pointing down an the lower ones pointing up. These nozzle assemblies are relatively vertically displaceable, normally with the upper one movable and the lower one stationary. In addition the nozzle assemblies each define a surface corresponding to the respective face of the shaped glass sheet. Thus blasts of quenching air can be directed at the faces of the sheet from very close, making it possible to create in the glass prestresses between the surfaces and interior that give the beneficial qualities of tempered glass.
It is necessary to provide special-duty loading and unloading conveyors for such a system. The loading conveyor moves the hot shaped sheet that is to be tempered into position between the nozzle assemblies. The unloading conveyor takes the quench-tempered shaped sheet out from between the nozzle assemblies and hands it over to a further stage in the manufacturing process.
The above-cited European patent document describes a system wherein the loading conveyor includes an loading ring on which the outer periphery of the workpiece can rest, whereas the unloading conveyor has an unloading ring of similar shape. The loading conveyor manipulates the workpiece into the quenching station, that is between the nozzle assemblies, while holding it in the loading ring. The pressure to the lower nozzle assembly is increased and/or the pressure to the upper nozzle assembly is decreased to lift the workpiece up off the loading conveyor and hold it against the upper assembly. The loading ring can then move out of the station and the unloading ring can swing it, whereupon appropriate pressure changes can drop the sheet back down into the unloading ring.
Such a ring arrangement does not provide sufficient support for the workpiece, so it can deform during the quenching operation. Any deformations can ruin the optical quality of the piece, forcing it to be rejected. In addition the prestressing thus created in the glass is distributed evenly overall, or at least randomly distributed. Such prestressing is undesirable, it generally being agreed that for best fracturing action it is desirable to prestress in a pattern, so that on fracturing when all stresses break up the whole piece the breaking follows a dense pattern.